Radiation heat collection device

ABSTRACT

This invention relates to a device that comprises at least one collection unit ( 11 ), equipped with a collection tube ( 21 ) placed on supports ( 23 ), which is formed by an inner absorber tube ( 31 ) shaped as a continuous tube and an outer envelope tube ( 33 ). The collection unit ( 11 ) also comprises reflectors ( 15 ) that direct the radiation toward the collection tube ( 21 ). Moreover, the device comprises means ( 41, 43 ) designed to maintain the collection tube ( 21 ) space between the absorber tube ( 31 ) and the envelope tube ( 33 ) at a pressure of between 5·10 −1 -5·10 −2  mbar. The main advantages of the invention include the reduction in the breaking of glass due to the lower stresses to fatigue, an increase in the effective collection surface (97%-98%) and active management of the vacuum, which makes it possible to monitor the evolution thereof at all times.

FIELD OF THE INVENTION

This invention relates to a radiation heat collection device and, more particularly, to a heat collection device which uses a parabolic reflector that focuses the solar heat radiation onto an absorber tube.

BACKGROUND OF THE INVENTION

The solar heat collection devices that use cylindrical-parabolic concentrators known in the state of the art are based on the developments arising from the construction of thermoelectric plants called “Segs”, which were implemented in the United States in the 1980s.

The known designs use structures that act as supports for the reflector elements that make up the parabolic reflector profile and the absorber tube located on the theoretical focal line of the parabolic cylinder formed by the reflectors. These structures are usually formed by inter-connected modules and equipped with an orientation mechanism that makes it possible to collect the maximum possible radiation by following the sun.

The reflector elements may be composed of different materials, made using different processes and supported in different ways, but, in any event, the objective is to obtain the maximum possible reflectivity and the maximum geometric precision such that, following the optical laws of reflection and refraction, the reflected beam interception deviation with respect to the theoretical focal point is as small as possible.

One of the main elements of the device is the tube in charge of absorbing the maximum possible energy of that reflected from the reflector surface and transmitting it as efficiently as possible to the heat-transfer fluid used. In order to prevent losses by thermal convection, the absorber tube, normally made of a metallic material with a suitable selective-layer coating, is surrounded by a glass envelope tube, and the intermediate space is subjected to high vacuum, which requires hermetic glass-metal joints and high-quality metal-metal welds subjected to vacuum. U.S. Pat. No. 6,705,311 describes specific solutions in this regard.

On the other hand, configuration of the device with completely airtight segments with high vacuum requirements has favoured the use of Getter systems for hydrogen absorption, such as that disclosed in US 2004134484.

The disadvantages of the solar collection devices known in the state of the art include the following:

-   -   High cost of the absorber tubes.     -   Breaking of the glass-metal joints, with the consequent loss of         vacuum and, therefore, of yield.     -   Breaking of the glass tube at the joint areas.     -   Need to perform costly temperature and vacuum degasification         processes that allow to activate the getter system.     -   Undetectability of the loss of vacuum in a tube or great         uncertainty of the tracers specified, with low confidence         regarding the information prior to the failure.     -   Possible saturation of H2 in the getter system through time for         temperatures below the working temperature, which would cause a         significant loss of yield.     -   Impossibility to easily measure the vacuum during the device's         operating life.     -   Need to use materials such as glass for the envelope tube and         glass-metal joints due to the high level of vacuum required.     -   High replacement cost in the event of breakage.     -   Impossibility to repair the replaced element.     -   Limited useful surface due to the need to absorb the         differential dilations between the inner tube and the outer         cover by means of bellows and connecting elements.

This invention is intended to overcome these disadvantages.

SUMMARY OF THE INVENTION

One object of this invention is to provide robust, controllable thermoelectric plants that ensure a minimum level of inoperativity, caused by the breaking of their components and the need for complex maintenance operations to replace them, during their operating life.

Another object of this invention is to provide thermoelectric plants that allow for flexible, cost-optimized production and exploitation.

Another object of this invention is to provide thermoelectric plants that make it possible to increase the interception factor and, consequently, the yield thereof.

Another object of this invention is to provide thermoelectric plants that make it possible to increase the effective surface in the absorber element, and, consequently, the yield thereof.

These and other objects are achieved by means of a heat collection device that comprises at least one collection unit equipped with a collection tube, formed by an inner absorber tube and an outer envelope tube, and reflectors that direct the radiation to the collection tube, wherein:

-   -   The inner absorber tube is shaped as a continuous tube.     -   The outer envelope tube is formed by multiple segments united by         joints that include support and slide means for the inner         absorber tube.     -   The collection tube supports have a rigid structure and are         fixed to the collection unit's bearing structure.     -   There are means available to maintain a pressure of between         5·10⁻¹-5·10⁻² mbar in the collection tube space between the         inner absorber tube and the outer envelope tube when the device         is operative.

In a preferred embodiment of this invention, the heat collection device collects solar radiation by means of a parabolic reflector, amongst other elements.

In a preferred embodiment of this invention, said heat collection device also comprises means for introducing gas into the collection tube which allow for hydrogen drag. Jointly with the vacuum production means, this leads to a device that makes it possible to monitor and control the vacuum level and the presence of hydrogen in the intermediate space between the inner absorber tube and the outer envelope tube, which contributes to optimizing the yield of the device.

In a preferred embodiment of this invention, the joints of all the segments making up the outer envelope tube are fixed to the collection tube supports. This provides for a very robust heat collection device.

In a preferred embodiment of this invention, part of the joints of all the segments making up the outer envelope tube are supported in a displaceable manner on the collection tube supports. This provides for a heat collection device wherein maintenance operations are reduced.

In a preferred embodiment of this invention, the outer envelope tube is made of glass, with a thickness of between 2.5 and 3.5 mm. This provides for a heat collection device with a very optically efficient collection tube.

In a preferred embodiment of this invention, the outer envelope tube is made of Polymethylmethacrylate (PMMA). This provides for a heat collection device with a less expensive collection tube.

In a preferred embodiment of this invention, the inner absorber tube has an oval cross-section. This provides for a heat collection device with a collection tube that improves the utilization of solar radiation.

In different preferred embodiments of this invention, inner absorber tubes with a circular cross-section are used, with a diameter between 70 and 90 mm and/or configurations with focal lengths Fl between 1,700 mm and 1,900 mm. This provides for more efficient heat collection devices.

In different preferred embodiments of this invention, the segments of the outer envelope tube have a length of between 4 and 6 m, and the segments of the inner absorber tube have a length of between 12 and 16 m. This provides for easy-to-assemble heat collection devices.

Other characteristics and advantages of this invention will arise from the detailed description of an embodiment that illustrates the object thereof in relation to the accompanying figures.

DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view of a collection unit of a heat collection device in accordance with this invention.

FIG. 2 schematically shows the directioning of radiation toward the collection tube of a solar heat collection device in accordance with this invention.

FIG. 3 is a perspective view of the final part of the collection tube of a heat collection device in accordance with this invention.

FIG. 4 is a schematic diagram that illustrates the vacuum formation process in a heat collection device in accordance with this invention formed by four collection units.

FIG. 5 is a schematic diagram that illustrates the gas drag in a heat collection device in accordance with this invention formed by four collection units.

FIG. 6 is a diagram that shows the relationship between heat losses and pressure in the intermediate space between the outer envelope tube and the inner absorber tube for air and hydrogen.

FIG. 7 is a figure similar to FIG. 2, except that the inner absorber tube of the collection tube has an oval cross-section.

FIG. 8 is a perspective view of a first embodiment of the segment joint of the outer envelope tube in a heat collection device in accordance with this invention.

FIG. 9 is a partial-section perspective view of said first embodiment of the joint, and FIGS. 9 a and 9 b are detailed views of two areas of the joint.

FIG. 10 is a cross-section view of the insulation crown of the joint illustrated in FIG. 9.

FIG. 11 is a partial-section perspective view of a second embodiment of the segment joint of the outer envelope tube in a heat collection device in accordance with this invention.

FIG. 12 is a partial view of a longitudinal section of the joint of FIG. 11.

FIG. 13 is a perspective view of the insulation crown of the joint illustrated in FIGS. 11 and 12.

FIG. 14 is a cross-section view of the joint illustrated in FIGS. 11-13.

FIG. 15 is a partial elevation view of a heat collection device in accordance with this invention using a third embodiment of the segment joint of the outer envelope tube.

FIG. 16 is a detailed view of the area of FIG. 17 with two joints and an intermediate bellows.

FIG. 17 is a partial-section perspective view of the joint of FIGS. 15-16.

FIG. 18 is a partial view of a longitudinal section of the joint of FIG. 17.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 represents a collection unit 11 of a heat collection device formed by two semi-collectors 13, 13′ with reflectors 15, 15′ that reflect the radiation toward collection tube 21, which extends along the focal line of said reflectors 15, 15′ and is placed on supports 23 fixed to the bearing and guiding structure of collection unit 11. Central area 17 of said structure also contains components 19 of the vacuum and gas drag systems whereto we will refer further below. Oil or another fluid circulates through collection tube 21; it is heated by the radiation and transmitted to a heat exchanger or directly to a turbine in the event that Water-Steam is used as the heat-transfer fluid in order to, for example, produce electrical energy, or to another device designed to utilize the fluid's calorific energy. As shown in FIG. 2, collection tube 21, which receives the beams reflected by a reflector 15, comprises an inner absorber tube 31 and an outer envelope tube 33 with an intermediate space that, as we shall see, is subjected to controlled vacuum when collection unit 11 is operative.

In the installations known in the state of the art, inner tube 31 has a diameter of about 70 mm and the focal length, Fl, is located at about 1,700 mm; as is well known, these magnitudes are determined by the characteristics of the parabolic concentrators for purposes of optimizing the interception factor of the reflected beams.

Inner absorber tube 31 and outer envelope tube 33 may be made of the same type of materials used in the thermoelectric plants known in the state of the art, typically metal coated with selective layers for the absorber tube and glass for the envelope tube.

In a preferred embodiment of the invention, the thickness of outer envelope tube 33, made of glass, is between 2.5 and 3.5 mm, and, preferably, 3 mm for borosilicates.

Outer envelope tube 33 may also be made of other suitable transparent materials, such as Polymethylmethacrylate (PMMA).

In a preferred embodiment of the invention, inner absorber tube 31 is formed by welded segments with lengths of between 12 and 16 meters, and outer envelope tube 33 is formed by segments with lengths of between 4 and 6 meters. The segment joints of outer envelope tube 33 are placed on supports 23 and shaped, as shown further below, to fulfill a three-fold purpose: that the intermediate space between outer envelope tube 33 and inner absorber tube 31 may be subjected to vacuum, that the segments of outer envelope tube 33 may dilate/contract due to an increase/reduction in the temperature without compromising its integrity nor the airtightness of said intermediate space, and that they provide a support and slide base for inner absorber tube 31 such that it may freely dilate/contract thereon.

Consequently, inner absorber tube 31 freely dilates independently from outer envelope tube 33 and the dilations of the latter are absorbed at the joints of the segments that compose it; therefore, the intermediate space between both, which extends along the entire device, may act as a vacuum chamber. In the final part of each collection tube 21, a bellows 25 (see FIG. 3) accumulates the total dilations of inner absorber tube 31 with respect to the last support 23, without transmitting a cyclised load to envelope tube 33. Thus, bellows 25 are the closing elements of the above-mentioned vacuum chamber.

FIGS. 4 and 5 schematically illustrate the vacuum and gas drag systems of a heat collection device in accordance with the invention composed of four collection units 11, each formed by two semi-collectors 13, 13′.

The vacuum system comprises a central vacuum pump 41 and two end vacuum pumps 43 for each collection unit 11. FIG. 4 illustrates the air flow with all the pumps 41, 43 under operating conditions.

The gas drag system comprises a gas-dispensing element 45 connected to the central part of each collector 11. FIG. 5 illustrates the gas flow when the system is started with dispensing elements 45 and end pumps 43 under operating conditions.

The operation of the heat collection device in accordance with this invention may be described as follows:

The device receives direct radiation by orientation toward the radiation source in one or two axes, preferably one axis for thermoelectric production applications, by heating the surface of inner absorber tube 31, which causes heating of the oil circulating inside. Right before the device is oriented toward the radiation heat source, vacuum pumps 41, 43 begin to create a vacuum in the intermediate space between inner absorber tube 31 and outer envelope tube 33 until a maximum pressure, of between 5·10⁻¹ and 5·10⁻² mbar, is achieved along the entire length. This pressure is sufficient to almost completely eliminate heat losses through the vacuum chamber when the material inside it is primarily air; this is not the case when the gas is hydrogen, as may be seen in FIG. 6, in curves 27, 29 that show the relationship between heat losses and pressure for, respectively, air and hydrogen. This Figure is taken from “Technical Report: Heat transfer analysis and Modeling of a Parabolic Trough Solar Receiver Implement in Engineering Equation Solver. R. Forristel”. The joints of outer envelope tube 33, whereto we will refer in more detail further below, make it possible to achieve that vacuum level, such that it is not necessary to implement high-vacuum tubes with hermetic seals, GTMS welds and Getter systems. As the heating takes place, the temperature of inner absorber tube 31 increases to its maximum, between 400° C. and 600° C., thereby increasing the length, sliding along the support and positioning means placed on the joints, which are designed for point contact. Supports 23 of outer envelope tube 33 are fixed and, therefore, will remain in the specified position during said operation.

During operation of the heat collection device, the vacuum action will be maintained only as long as necessary, due to the degasification produced by the surface of the components of inner absorber tube 31. Everyday operation will provide for a direct degasification system in the installation, such that it is not necessary to perform said operation in each inner absorber tube prior to assembling it. In the event that the heat losses increase due to the presence of hydrogen, a drag operation will be performed which consists of introducing a given gas, such as dry air, CO2 or others with a conduction coefficient equal to or lower than that of air, into the vacuum chamber by means of dispensing elements 45, controlling either the flow rate, the temperature of the gas introduced, or both, in order to prevent potential thermal shocks, in the event that glass outer envelope tubes 33 are used, and the subsequent discharge by means of the vacuum system. The previous operation will make it possible to control, whenever necessary, the partial pressure of hydrogen and, therefore, the rate of heat loss due to the existing gas mixture and the vacuum chamber pressure. If outer envelope tube 33 is made of a suitable transparent plastic material, such as Polymethylmethacrylate (PMMA), there is no risk of said thermal shocks.

The vacuum and gas drag systems are electronically programmable and controllable systems designed to operate the dynamic vacuum on the basis of the needs at each time, without the complexity that the presence of thousands of tubes designed to last 25 years with 10⁻⁴ mbar vacuums entails.

The independence between inner absorber tube 31 and outer envelope tube 33 also makes it possible for inner tube 31 to have, as shown in FIG. 7, an oval geometry with the major semi-axis oriented in the direction of the axis of the parabola, in order to improve the interception factor of the reflected beams, particularly at the ends of parabola 15, where there is the maximum probability of separation in the focal area. Due to the difference in the distance from the parabola to the focus at the different points thereof, the probability distribution of reflected beams at a given point of the parabola translates into an angular aperture β, the linear aperture “a” whereof will be a function of distance “d” to the focus of the point in question. FIG. 7 shows the variation of linear aperture a1 and a2 for points p1 and p2 positioned around the centre and the end of parabola 15 at distances d1 and d2, respectively. The oval geometry of inner absorber tube 31 makes it possible to increase the yield of the system using a smaller quantity of material than that necessary to cover the same area with a circular geometry. Similarly, the lateral section subject to losses by radiation will be lower than the circular equivalent.

As regards the segment joints of outer envelope tube 33, below we will describe a first embodiment of this invention, in accordance with FIGS. 8-10.

Each joint 51 comprises:

-   -   Two flanges 53, 53′ with axial tightening means (FIGS. 8 and 9         do not show the typical bolts used in this type of flanges, but         they do show the flange openings), the configuration whereof         includes sloping surfaces 55, 55′, which co-operate with         enlarged edges 57, 57′ of the ends of segments 35, 35′ of         envelope tube 33 through contact elements 59, 59′; these have a         high elasticity and a low hardness in order to prevent the         generation of local stresses in the glass.     -   A ring 61, placed between flanges 53, 53′, with two hermetic         sealing bands 63, 63′ designed to maintain the vacuum, with a         fixation base 69 on a support 23.     -   An insulation crown 62 bound to ring 61, and support and slide         means for inner absorber tube 31, consisting of inner radial         teeth 64 which provide point contacts with inner absorber tube         31 and allow it to slide with a suitable tribologic behaviour         that minimizes friction and wear. Insulation crown 62 also         fulfils a thermal insulation function in order to minimize         losses by conduction. Preferably, it is made of a ceramic         material.

As already mentioned, ends 55, 55′ of segments 35, 35′ of envelope tube 33 have an enlarged configuration in order to collaborate with flanges 53, 53′ and, furthermore, their final area 56 is protected against concentrated IR and solar radiation.

This configuration of joint 51 takes advantage of the dilation of outer envelope tube 33 due to the increase in temperature, since it generates an added compression effect on sealing bands 63, due to the fixed position, as base 69 of ring 61 is fixed to support 23.

The thermal insulation system causes the temperatures in the areas of sealing bands 63, 63′ not to exceed 100° C.; consequently, different materials with the adequate specifications may be used. The mean temperature of outer envelope tube 33 will be a function of the material used, but, in any event, the limited dilation thereof will be absorbed by sealing bands 63, 63′. The tightening and contact elements of segments 35, 35′ of outer envelope tube 33 must have sufficient flexibility to maintain them in a radial position during the displacements thereof. Sloping surfaces 55, 55′ of flanges 53, 53′ have been designed for this purpose. The angle Ω to be used is a function of the elasticity of the tightening system and the load per empty weight to be borne by said system, which will generate a normal component that is a function of said angle Ω.

Furthermore, insulation crown 62 is intended to connect the vacuum or insulation chamber with the adequate conductivity by means of a design that allows for an adequate difference in pressure between the endpoints of the vacuum chamber defined between absorber tube 31 and outer tube 33, the maximum pressure being lower than 5*10⁻¹ mbar at all times. To this end, the hydraulic diameter of the cross-section must be at least greater than 75% of that corresponding to the annular section defined by the inner diameter of outer envelope tube 33 and the outer diameter of inner absorber tube 31.

In a second embodiment, as may be observed in FIGS. 11-14, joints 81 comprise:

-   -   Two clamp-flanges 83, 83′ with radial tightening means (not         shown) that join to frames 84, 84′, which are fixed on supports         23.     -   An insulation crown 85 with, on the one hand, slots 87, 87′         designed to receive the pair of segments 35, 35′ of outer         envelope tube 33, with sufficient clearance to this end, and, on         the other hand, support and slide means for inner absorber tube         31 consisting of inner radial teeth 86 which provide point         contacts for inner absorber tube 31 and allow it to slide with a         suitable tribologic behaviour that minimizes friction and wear.         Insulation crown 85 also fulfils a thermal insulation function         in order to minimize losses by conduction. Preferably, it is         made of a ceramic material.     -   Two pairs of sealing bands 89, 89′ placed on lateral openings of         insulation crown 85, on the edges of said slots 87, 87′, such         that said segments 35, 35′ remain between them and,         consequently, a vacuum is maintained in the intermediate space         between inner absorber tube 31 and outer envelope tube 33.

Between flanges 83, 83′ and insulation crown 85, an intermediate part 88 is placed, made of an elastomer material designed to homogenize the effect of tightening flanges 83, 83′ on insulation crown 85. The dilations of segments 35, 35′ are absorbed with small deformations of sealing bands 89, 89′.

Although joints 51 and 81 are constructively different, they may be considered to be functionally equivalent.

In a third embodiment of the invention, illustrated in FIGS. 15-18, a group of two segments 36, 37 of outer envelope tube 33 is united by means of a joint 95 fixed onto a support 23 and the ends of that group are united to other adjacent groups by means of pairs of joints 95′ supported in a displaceable manner on a support 23 with an intermediate bellows device 97 which, in turn, facilitates welding of the components of inner absorber tube 31 between two joints 95′ by the contraction of bellows 97 and the subsequent assembly thereof.

As observed in FIG. 15, the group formed by segments 36 and 37, which has a length L of, for example, 12 m, is centrally united by a joint 95 fixed onto a support 23 and, on the ends, by joints 95′ supported in a displaceable manner on supports 23. In turn, end joints 95′ of each group are connected by means of a bellows 97. The arrows, f, indicate the displacements of the end joints as a consequence of the dilations of segments 36, 37 of outer envelope tube 33.

The axial dilation of segments 36, 37 of a glass envelope tube 23 may be calculated to be about 2 mm every 6 m for a maximum temperature difference of 100° C.; for this reason, supports 23 of the ends of each group of segments 36, 37 may allow for displacements of joints 95′ of about 3 mm.

Bellows 97 fulfils the three following functions:

-   -   It absorbs the axial movement of adjacent glass segments, which,         on the basis of the above, may be considered to be less than 6         mm.     -   It gives continuity to the vacuum chamber defined between inner         absorber tube 31 and outer envelope tube 33.     -   It makes it possible to disassemble the components of the inner         absorber tube in order to repair them, if necessary.

Joints 95, 95′ comprise:

-   -   Two flanges 101, 101′, with first tightening means 103, 103′ and         second tightening means 104, 104′ consisting of encapsulated         traction springs, the configuration whereof includes sloping         surfaces 105, 105′, which co-operate with enlarged edges 107,         107′ of the ends of segments 36, 37 of envelope tube 33 through         contact elements 109, 109′, made of a material with a medium         hardness and a low elasticity constant, such as Teflon.     -   A ring 111, placed between flanges 101, 101′ with two hermetic         sealing bands 113, 113′ designed to maintain the vacuum, with a         support base 119 on a support 23. Due to the action of         tightening means 103, 103′; 104, 104′, sealing bands 113, 113′,         which are made of an elastomer material with suitable coatings         to maintain their characteristics during their estimated         lifetime, always have a pre-determined degree of compression,         for example 20% of the diameter thereof, regardless of the         working temperature of the heat-transfer fluid and, therefore,         of the temperature of outer envelope tube 33 and of inner         absorber tube 31. In joints 95, support base 119 is united to         support 23 and, in joints 95′, it is supported on support 23,         such that it may slide thereon.     -   An insulation crown 112 joined to ring 111 with support and         slide means for inner absorber tube 31, consisting of inner         radial teeth 114 that provide point contacts for inner absorber         tube 31 and allow it to slide with a suitable tribologic         behaviour that minimizes friction and wear. It also includes         inner protectors 121 in the form of a “cooking pan”, in order to         protect sealing bands 113, 113′ and contact elements 109, 109′         against the concentrated radiation produced by the reflector         mirrors which might reach them as a result of optical errors.         Insulation crown 112 also fulfils a thermal insulation function         in order to minimize losses by conduction. Preferably, it is         made of a ceramic material.

The collection device in accordance with this invention makes it possible to surpass the magnitudes of the parabolic solar concentrators known in the state of the art in, at least, the following aspects:

-   -   The diameter of circular absorber tubes 31, which may be between         70-90 mm and equivalent dimensions in the case of absorber tubes         with an oval cross-section or tubes with other types of         cross-sections.     -   Focal length Fl, which may be between 1,700-1,900 mm.

This is basically due to the greater optical precision that may be achieved with rigid supports 23 and the absence of significant movements of collection tube 21. The current systems absorb axial dilations by the rotating displacement of the rigid supports, thereby causing a loss of position of the absorber tube with respect to the theoretical position, of 12 mm on average and 24 mm maximum, exclusively calculated on the basis of the design concept, without taking into consideration manufacturing and assembly precisions.

Furthermore, it allows to improve the useful collection surface of collection tube 21, which may be estimated to be between 97%-98% of the total length, thanks to the absence of both glass-metal joints and intermediate bellows for the compensation of thermal dilations, estimated to be 25 mm every 6 m as the differential dilation between the inner tube and the outer cover, as a consequence of using a continuous absorber tube 31.

The advantages of the heat collection device in accordance with the invention with respect to the solar heat collection devices known in the state of the art include the following:

-   -   Absence of GTMS welds.     -   Less shaded space per meter. Greater effective surface.     -   Absence of Getter for hydrogen control.     -   Potentially higher optical yield due to the possibility of fixed         supports. Dilation absorbed in the end.     -   Possibility to use absorber tubes with a non-cylindrical         geometry and, in particular, an oval geometry, which improve the         yield.     -   Ease of assembly and process. Gasifications are not required for         high vacuum levels.     -   No need for highly airtight joints.     -   Reduced breaking of glass due to lower stresses to fatigue.     -   Possibility to reduce the thickness of the glass outer envelope         tube since GTMS welds are not required and lower stresses are         borne at the ends. Improved optical yield due to better         transmittance.     -   Active management of the vacuum and the presence of hydrogen         make it possible to know the location at each time and monitor         the evolution thereof, unlike in the case of hermetically closed         systems.     -   Ease of manufacturing.     -   Reduced vacuum requirements.     -   Elimination of the need to use Getter systems and vacuum control         elements in the closed chamber.     -   Greater precision of the position of the absorber tube with         respect to the theoretical focal point, since it may move on         fixed supports during dilation.

Regarding the embodiments of the invention described, those modifications included within the scope defined by the following claims may be introduced. 

1. Radiation heat collection device that comprises at least one collection unit (11) equipped with a collection tube (21) placed on supports (23) and reflectors (15), wherein the collection tube is formed by an inner absorber tube (31) and an outer envelope tube (33), the reflectors (15) direct the radiation toward the collection tube (21), and wherein said radiation heat collection device comprises means (41, 43) designed to maintain the collection tube (21) space between the inner absorber tube (31) and the outer envelope tube (33) at a pressure of between 5×10⁻¹-5×10⁻² mbar when the device is operative.
 2. Device, as claimed in claim 1, wherein the inner absorber tube (33) is shaped as a continuous tube.
 3. Device, as claimed in claim 1, wherein the outer envelope tube (33) is formed by multiple segments united by joints (51, 81, 95, 95′) that include support and slide means (65) for the inner absorber tube (31).
 4. Device, as claimed in claim 1, wherein the collection tube (21) supports (23) are shaped as a rigid structure and fixed to a bearing structure of the collection unit (11).
 5. Device, as claimed in claim 19, wherein each collection unit (11) is formed by two semi-collectors (13, 13′) and are connected by vacuum pumps at a central part (41) and at an end (43).
 6. Device, as claimed in claim 5, wherein the device further comprises an element (45) that is connected to the central part of each collection unit (11), designed for the introduction of a gas that allows to drag the hydrogen present in the collection tube (21) space between the inner absorber tube and the outer envelope tube (33), in collaboration with the vacuum pumps (43) connected to the ends of each collection unit (11).
 7. Device, as claimed in claim 3, wherein said joints (51) comprise: flanges (53, 53′) with co-operating means (55, 59; 55′, 59′) with the ends (57, 57′) of each pair of segments of the outer envelope tube (33); a ring (61) placed between said flanges (53, 53′), fixed to a support (23); sealing bands (63, 63′) placed in openings of said ring (61) adjacent to said ends (57, 57′); an insulation crown (62) with inner radial teeth (64) designed for the support and sliding of the inner absorber tube (31) joined to said ring (61); and axial tightening means designed to bring said ends (57, 57′) close to the ring (61).
 8. Device, as claimed in claim 3, wherein said joints (81) comprise: flanges (83, 83′) fixed to a support (23); an insulation crown (85) with lateral slots (87, 87′) designed for the reception of each pair of segments (35, 35′) of the outer envelope tube (33) between pairs of sealing bands (89, 89′); and inner radial teeth (86) designed for the support and sliding of the inner absorber tube (31) and radial tightening means for said flanges (83, 83′).
 9. Device, as claimed in claim 1, wherein each collection unit is formed by multiple groups of segments (36, 37) of the outer envelope tube (33), wherein the segments (36, 37) of each group are united by joints (95) fixed to supports (23), and the ends of each group are united by pairs of joints (95′) supported in a displaceable manner on supports (23) with an intermediate bellows device (97), said joints (95, 95′) comprising: flanges (101, 101′) with co-operating means (105, 109; 105′, 109′) with the ends (107, 107′) of each pair of segments (36, 37) of the outer envelope tube (33); a ring (111) placed between said flanges (101, 101′); sealing bands (113, 113′) placed in openings of said ring (112) adjacent to said ends (107, 107′); an insulation crown (112) with inner radial teeth (114) designed for the support and sliding of the inner absorber tube (31) joined to said ring (111); and means designed to maintain said ends (107, 107′) and the ring (111) immobilized.
 10. Device, as claimed in claim 1, wherein said outer envelope tube (33) is made of glass, or of polymethylmethacrylate (PMMA).
 11. Device, as claimed in claim 10, wherein the outer envelope tube (33) is made of glass and wherein the thickness of said outer envelope tube is between 2.5 and 3.5 mm.
 12. Device, as claimed in claim 1, wherein the inner absorber tube (31) has a circular cross-section and a diameter of between 70 and 90 mm.
 13. Device, as claimed in claim 1, wherein said inner absorber tube (31) has an oval cross-section.
 14. Device, as claimed in claim 1, wherein the useful collection surface of the collection tube (21) is between 97%-98% of its total length.
 15. Device, as claimed in claim 1, wherein the focal length is between 1,700 mm and 1,900 mm.
 16. Device, as claimed in claim 1, wherein the length of the segments of the outer envelope tube (33) united by the joints (51, 81, 95, 95′) is between 4 and 6 m.
 17. Device, as claimed in claim 1, wherein the length of the segments of the inner absorber tube (31) joined by welds is between 12 and 16 m.
 18. Device, as claimed in claim 1, wherein the solar radiation is collected by means of parabolic reflectors.
 19. Device, as claimed in claim 1, wherein said means comprise vacuum pumps.
 20. Device, as claimed in claim 10, wherein the means designed to maintain said ends (107, 107′) and the ring (111) immobilized include encapsulated traction springs (104, 104′). 